Title: The importance of calorimetry for highly-boosted jet substructure

Abstract

Here, jet substructure techniques are playing an essential role in exploring the TeV scale at the Large Hadron Collider (LHC), since they facilitate the efficient reconstruction and identification of highly-boosted objects. Both for the LHC and for future colliders, there is a growing interest in using jet substructure methods based only on charged-particle information. The reason is that silicon-based tracking detectors offer excellent granularity and precise vertexing, which can improve the angular resolution on highly-collimated jets and mitigate the impact of pileup. In this paper, we assess how much jet substructure performance degrades by using track-only information, and we demonstrate physics contexts in which calorimetry is most beneficial. Specifically, we consider five different hadronic final states - W bosons, Z bosons, top quarks, light quarks, gluons - and test the pairwise discrimination power with a multi-variate combination of substructure observables. In the idealized case of perfect reconstruction, we quantify the loss in discrimination performance when using just charged particles compared to using all detected particles. We also consider the intermediate case of using charged particles plus photons, which provides valuable information about neutral pions. In the more realistic case of a segmented calorimeter, we assess the potential performance gains frommore » improving calorimeter granularity and resolution, comparing a CMS-like detector to more ambitious future detector concepts. Broadly speaking, we find large performance gains from neutral-particle information and from improved calorimetry in cases where jet mass resolution drives the discrimination power, whereas the gains are more modest if an absolute mass scale calibration is not required.« less

@article{osti_1416558,
title = {The importance of calorimetry for highly-boosted jet substructure},
author = {Coleman, Evan and Freytsis, Marat and Hinzmann, Andreas and Narain, Meenakshi and Thaler, Jesse and Tran, Nhan and Vernieri, Caterina},
abstractNote = {Here, jet substructure techniques are playing an essential role in exploring the TeV scale at the Large Hadron Collider (LHC), since they facilitate the efficient reconstruction and identification of highly-boosted objects. Both for the LHC and for future colliders, there is a growing interest in using jet substructure methods based only on charged-particle information. The reason is that silicon-based tracking detectors offer excellent granularity and precise vertexing, which can improve the angular resolution on highly-collimated jets and mitigate the impact of pileup. In this paper, we assess how much jet substructure performance degrades by using track-only information, and we demonstrate physics contexts in which calorimetry is most beneficial. Specifically, we consider five different hadronic final states - W bosons, Z bosons, top quarks, light quarks, gluons - and test the pairwise discrimination power with a multi-variate combination of substructure observables. In the idealized case of perfect reconstruction, we quantify the loss in discrimination performance when using just charged particles compared to using all detected particles. We also consider the intermediate case of using charged particles plus photons, which provides valuable information about neutral pions. In the more realistic case of a segmented calorimeter, we assess the potential performance gains from improving calorimeter granularity and resolution, comparing a CMS-like detector to more ambitious future detector concepts. Broadly speaking, we find large performance gains from neutral-particle information and from improved calorimetry in cases where jet mass resolution drives the discrimination power, whereas the gains are more modest if an absolute mass scale calibration is not required.},
doi = {10.1088/1748-0221/13/01/T01003},
journal = {Journal of Instrumentation},
number = 01,
volume = 13,
place = {United States},
year = {2018},
month = {1}
}

The International Large Detector (ILD) is a concept for a detector at the International Linear Collider, ILC. The ILC will collide electrons and positrons at energies of initially 500 GeV, upgradeable to 1 TeV. The ILC has an ambitious physics program, which will extend and complement that of the Large Hadron Collider (LHC). The ILC physics case has been well documented, most recently in the ILC Reference Design Report, RDR. A hallmark of physics at the ILC is precision. The clean initial state and the comparatively benign environment of a lepton collider are ideally suited to high precision measurements. Tomore » take full advantage of the physics potential of ILC places great demands on the detector performance. The design of ILD, which is based on the GLD and the LDC detector concepts, is driven by these requirements. Excellent calorimetry and tracking are combined to obtain the best possible overall event reconstruction, including the capability to reconstruct individual particles within jets for particle flow calorimetry. This requires excellent spatial resolution for all detector systems. A highly granular calorimeter system is combined with a central tracker which stresses redundancy and efficiency. In addition, efficient reconstruction of secondary vertices and excellent momentum resolution for charged particles are essential for an ILC detector. The interaction region of the ILC is designed to host two detectors, which can be moved into the beam position with a 'push-pull' scheme. The mechanical design of ILD and the overall integration of subdetectors takes these operational conditions into account. The main features of ILD are outlined below. The central component of the ILD tracker is a Time Projection Chamber (TPC) which provides up to 224 precise measurements along the track of a charged particle. This is supplemented by a system of Silicon (Si) based tracking detectors, which provide additional measurement points inside and outside of the TPC, and extend the angular coverage down to very small angles. A Si-pixel based vertex detector (VTX) enables long lived particles such as b- and c-hadrons to be reconstructed. This combination of tracking devices, which has a large degree of redundancy, results in high track reconstruction efficiencies, and unprecedented momentum resolution and vertex reconstruction capabilities. One of the most direct measures of detector performance at the ILC is the jet-energy resolution. Precise di-jet mass reconstruction and separation of hadronically decaying W and Z bosons are essential for many physics channels. The ultimate jet energy resolution is achieved when every particle in the event, charged and neutral, is measured with the best possible precision. Within the paradigm of particle flow calorimetry, this goal is achieved by reconstructing charged particles in the tracker, photons in the electromagnetic calorimeter (ECAL), and neutral hadrons in the ECAL and hadronic calorimeter (HCAL). The ultimate performance is reached for perfect separation of charged-particle clusters from neutral particle clusters in the calorimeters. Thus, a highly granular calorimeter outside the tracker is the second key component of ILD. Sampling calorimeters with dense absorber material and fine grained readout are used. A tungsten absorber based electromagnetic calorimeter (ECAL) covers the first interaction length, followed by a somewhat coarser steel based sampling hadronic calorimeter (HCAL). Several ECAL and HCAL readout technologies are being pursued.« less

The Higgs boson is the only elementary particle predicted by the Standard Model (SM) that has not yet been observed experimentally. If it exists, it explains the spontaneous electroweak symmetry breaking and the origin of mass for gauge bosons and fermions. We test the validity of the SM by performing a search for the associated production of a Higgs boson and a W boson in the channel where the Higgs boson decays to a bottom-antibottom quark pair and the W boson decays to a charged lepton and a neutrino (the WH channel). We study a dataset of proton-antiproton collisions atmore » a centre-of-mass energy √s = 1.96 TeV provided by the Tevatron accelerator, corresponding to an integrated luminosity of 5.7 fb -1, and recorded using the Collider Detector at Fermilab (CDF).We select events consistent with the signature of exactly one charged lepton (electron or muon), missing transverse energy due to the undetected neutrino (MET) and two collimated streams of particles (jets), at least one of which is required to be identified as originating from a bottom quark. We improve the discrimination of Higgs signal from backgrounds through the use of an artificial neural network. Using a Bayesian statistical inference approach, we set for each hypothetical Higgs boson mass in the range 100-150 GeV/c 2 with 5 GeV/c 2 increments a 95% credibility level (CL) upper limit on the ratio between the Higgs production cross section times branching fraction and the SM prediction. Our main original contributions are the addition of a novel charged lepton reconstruction algorithm with looser requirements (ISOTRK) with respect the electron or muon tight criteria (TIGHT), as well as the introduction of a novel trigger-combination method that allows to maximize the event yield while avoiding trigger correlations and that is used for the ISOTRK category. The ISOTRK candidate is a high-transverse-momentum good-quality track isolated from other activity in the tracking system and not required to match a calorimeter cluster, as for a tight electron candidate, or an energy deposit in the muon detector, as for a tight muon candidate. The ISOTRK category recovers real charged leptons that otherwise would be lost in the non-instrumented regions of the detector. This allows the reconstruction of more W boson candidates, which in turn increases the number of reconstructed WH signal candidate events, and therefore improves the sensitivity of the WH search. For the TIGHT charged lepton categories, we employ charged-lepton-dedicated triggers to improve the rate of WH signal acceptance during data taking. Since there is no ISOTRK-dedicated trigger at CDF, for the ISOTRK charged lepton category we employ three MET-plus-jets-based triggers. For each trigger we first identify the jet selection where the trigger efficiency is flat with respect to jet information (transverse energy and direction of motion in the transverse plane for the two jets in the event) and then we parametrize the trigger efficiency as a function of trigger MET. On an event-by-event basis, for each trigger we compute a trigger efficiency as a function of trigger parametrization, trigger MET, jet information, trigger prescale and information about whether the trigger is defined or not. For the ISOTRK category we combine the three triggers using a novel method, which allows the combination of any number of triggers in order to maximize the event yield while avoiding trigger correlations. On an event-by-event basis, only the trigger with the largest efficiency is used. By avoiding a logical 'OR' between triggers, the loss in the yield of events accepted by the trigger combination is compensated by a smaller and easier-to-compute corresponding systematic uncertainty. The addition of the ISOTRK charged lepton category to the TIGHT category produces an increase of 33% in the WH signal yield and a decrease of 15.5% to 19.0% in the median expected 95% CL cross-section upper limits across the entire studied Higgs mass interval. The improvement in analysis sensitivity is smaller than the improvement in signal yield because the ISOTRK category has a smaller signal over background ratio than the TIGHT category, due to the looser ISOTRK reconstruction criteria. The observed (median expected) 95% CL SM Higgs upper limits on cross section times branching ratio vary between 2.39 x SM (2.73 x SM) for a Higgs mass of 100 GeV/c 2 to 31.1 x SM (31.2 x SM) for a Higgs mass of 150 GeV/c 2, while the value for a 115 GeV/c 2 Higgs boson is that of 5.08 x SM (3.79 x SM). The novel trigger combination method is already in use by several CDF analyses. It is applicable to any analysis that uses triggers based on MET and jets, such as supersymmetry searches at the ATLAS and CMS experiments at the Large Hadron Collider. In its most general form, the method can be used by any analysis that combines any number of different triggers.« less

Particle physics today is at a crossroads. The Standard Model (SM) has survived four decades of experimental scrutiny and spurred multiple landmark discoveries, culminating with the 2012 discovery of the Higgs boson at the Large Hadron Collider (LHC). Yet we know that this model is incomplete. We live in a universe dominated by matter and energy unaccounted for by the SM– and the Higgs boson itself is improbably light. It falls to the current generation of high-energy physicists to go beyond the model and enter a new era of discovery. Andeen is leading a new research group at the Universitymore » of Texas at Austin (UT-Austin) that is endeavoring to reveal novel physical phenomena through the discovery of new particles with the ATLAS detector at the LHC. A key element of our experience thus far has been collaborating closely with theorists to identify both interesting new physics models. In addition we work closely with electrical engineers in detector development. Our approach is therefore unique in its integration of theory, analysis and instrumentation, and is encapsulated in a three-pronged strategy. First is the essential task of analyzing our rapidly expanding dataset. I am directing my group in searches for vector-like quarks (VLQs). Should they exist, these hypothetical particles would indicate physics scenarios beyond the SM. Experimental constraints suggest VLQs decay preferentially to a third generation quark and a Higgs, W or Z boson. Today at 13 TeV, both multi-lepton (electron and muon) and jet substructure signatures are sensitive to identifying the VLQ decay products. The large Run 2 dataset (collected through 2018) of up to 150 fb -1 will allow us to simultaneously use multi-lepton signatures with jet-substructure in searches for new physics. UT-Austin is well positioned to be a leader in this area. Prof. P. Onyisi’s postdoctoral researchers and graduate students are searching for ttH signatures in three- and four-lepton channels and we benefit from their experience. We are also aided by the availability of the Texas Advanced Computing Center, which provides us with easy access to several high performance computing (HPC) systems. My previous experience in VLQ searches propels the work today. I have published both the first investigation into the single production of top partners (signatures of composite Higgs Models) of any LHC experiment 1 as well as one of the first analyses utilizing jet-substructure techniques to search for the single production of vector-like quarks. 2 Adding to this is my group’s work with the Liquid Argon (LAr) calorimeter, the primary instrument we use in detecting electrons. Second, my group is contributing to the operations of the LAr calorimeter while preparing to install and commission new electronic hardware (the Phase 1 upgrade) to this crucial component of the ATLAS detector. I have played a leading role in the development of a critical, radiation-hard, high-speed Analog-to-Digital Convertor (ADC) that is essential inselecting data collected by the LAr calorimeter. 3 This upgrade will improve the e!ciency with which we can select events in our detector. Postdoctoral researcher N. Nikiforou (based at CERN May 2016 onward) and graduate students (one based at CERN from the start of 2018 onward, with one additional graduate students from summer 2018) are growing our already deep experience in the operation of the LAr calorimeter to prepare for installation and commissioning of the new trigger readout in 2018-2019. My group will then be positioned to be the first to exploit the potential of the improved triggers in our multi-lepton searches for new physics. Third, a suite of preparations for a major detector upgrade, scheduled to be completed in 2024, are underway. My group is building hardware that will improve the electronic readout of the ATLAS detector. I am partnering with a group in the Electrical and Computer Engineering (ECE) Department at UT-Austin (Prof. N. Sun) to employ his work at the cutting edge of ADC research. We will support an ECE grad student for the layout of this design, and collaborate with engineering groups at Nevis Laboratory and the Electrical Engineering Department (Prof. P. Kinget) at Columbia University to integrate our ADC with gain selection circuits. We will provide this critical component of the ASIC at the challenging boundary of the analog and digital signals in the LAr calorimeter readout chain. Furthermore UT-Austin will lead the testing of this device during development and production (approximately 55,000 four-channel chips). This effort compliments my work as the deliverables manager for the LAr front-end ASICs for the US ATLAS HL-LHC project and strengthens the US commitment to that upgrade.In so doing, my group at the UT-Austin will be at the forefront of efforts to probe the structure of the universe to an unmatched degree both at the current LHC and with its future upgrades. Through both the ongoing analysis of data at the ATLAS detector and improvements to the detector itself, an answer to what lies beyond the SM may come within our grasp. My background in instrumentation and analysis allows my group to establish a singular research effort on the ATLAS experiment. In addition to answering fundamental questions about the subatomic world, the technologies developed could advance the broader scientific community and result in valuable spin-offs.« less

This thesis describes the effort being made to improve the Jet Energy Reconstruction as performed by the CDF international collaboration at the Tevatron collider. This experiment studies proton-antiproton interactions at a center of mass energy of 1.8 TeV. During the three years data taking period Run 1, from 1992 to 1995 the CDF experiment collected an amount of data corresponding to a total integrated luminosity of 110 pb{sup -1}. One of the major results obtained analyzing this data sample is the discovery of the top quark. In the year 2000 a new period of data taking, Run 11, will startmore » with a higher luminosity and a slightly higher center of mass energy giving us the chance to explore high energy physics even deeper. In preparation of this new run several upgrades are being made to adapt the CDF detector to the high luminosity foreseen and to improve its capabilities. Many signatures requested to trigger the detector aim at signaling a quark or a gluon in the final state. Unfortunately we are not able to measure quarks as free particles because they undergo a fragmentation process when turning into jets of particles. Thus it is of key importance to build up algorithms which reconstruct the energy of the initial parton starting from the jet informations. The description of the algorithm adopted till now will be given as an introduction to the new method being developed, that will be the main subject of this thesis. In Chapter I we will give a theoretical introduction on strong interactions to describe the mechanism to produce hadronic jets. In Chapter 2 we will describe some results from the experiment where the reconstruction of hadronic jets was important. Here we will also mention some important results which we think we can obtain during new the data taking period. We will give particular emphasis to those processes where an improved jet energy measured would bring to better results. In Chapter 3 we will give a description of the CDF detector including some more details on the elements which are relevant for jet energy reconstruction. The way of defining jets which has been used by CDF so far, will be the subject of chapter 4. Starting from the present CDF algorithm we studied the various problems which arise with jet reconstruction. Those problems can be grouped into two categories, the one including effects coming from physics and a second one including the effects due to a non-perfect resolution of our detector. In Chapter 5 the physics effects limiting jet energy reconstruction will be addressed. We will discuss the radiation of hard gluons both from initial state and final state partons and we will show how these problems are connected with jet definition algorithms. In Chapter 6 we will describe a new method to define jet energy making use of some detector informations which are not used in the present algorithm. The energy of each single calorimeter tower will be re-defined taking into account not only the energy released in the calorimeters, but also the informations on the shower development through it and the tracking informations coming from the Central Tracking Chamber. Finally, in Chapter 7 we apply the studies described above on photon+jet events collected during the run 1. The use of data is of key importance to claim that our corrections are working fine. We will show how a 30 % improvement in jet energy resolution, a major step towards better jet physics in Run 11, is obtained.« less

The identification of hadronically decaying heavy states, such as vector bosons, the Higgs, or the top quark, produced with large transverse boosts has been and will continue to be a central focus of the jet physics program at the Large Hadron Collider (LHC). At a future hadron collider working at an order-of-magnitude larger energy than the LHC, these heavy states would be easily produced with transverse boosts of several TeV. At these energies, their decay products will be separated by angular scales comparable to individual calorimeter cells, making the current jet substructure identification techniques for hadronic decay modes not directlymore » employable. In addition, at the high energy and luminosity projected at a future hadron collider, there will be numerous sources for contamination including initial- and final-state radiation, underlying event, or pile-up which must be mitigated. We propose a simple strategy to tag such "hyper-boosted" objects that defines jets with radii that scale inversely proportional to their transverse boost and combines the standard calorimetric information with charged track-based observables. By means of a fast detector simulation, we apply it to top quark identification and demonstrate that our method efficiently discriminates hadronically decaying top quarks from light QCD jets up to transverse boosts of 20 TeV. Lastly, our results open the way to tagging heavy objects with energies in the multi-TeV range at present and future hadron colliders.« less